The use of ink-jet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today's ink-jet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality. In general, ink-jet print quality still falls short of that produced by more expensive technologies such as photography and offset or gravure printing. A surge in interest in ink-jet imaging (e.g., the rendition of pictures) has resulted in the need to produce near photographic quality printed images at a reasonable cost. The challenge remains to further improve the image quality of ink-jet printed images, without increasing their cost.
An ink-jet image is formed when a precise pattern of dots is ejected from a drop generating device known as a "printhead" onto a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an ink-jet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a "firing resistor," located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 .mu.m.times.35 .mu.m. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., ink-jet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
The tight tolerances of the nozzles, typically 30 to 40 .mu.m diameter require that the ink not clog the nozzles. Further, repeated firings of the resistor elements which must withstand many millions of firings over the life of the ink cartridge to be commercially practical, can result in fouling of the resistor elements and degrading pen performance. This build up of residue on the resistor elements is unique to ink-jet printers and is known as kogation and defined as the build-up of residue (koga) on the resistor surface.
In general, a successful ink for color ink-jet printing must be compatible with the ink-jet pen and printing system. Some of the required properties for the ink-jet ink include: good crusting resistance, the proper viscosity, the proper surface tension, little color-to-color bleed, rapid dry time, consumer safety, minimum coalescence, and low strike through. When the particular ink-jet application also includes special printing, such as imaging and rendition of pictures, such performance attributes are not only important on plain paper but also on special media, such as photobase glossy paper. Furthermore, the ink must be stable over the normal operating and storage temperatures.
Inks are known which possess one or more of the foregoing properties. However, few inks are known that possess all the foregoing properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations. Accordingly, investigations continue into developing ink formulations which have improved properties and which do not improve one property at the expense of the others. Thus, challenge remains to further improve the image quality of the ink-jet prints without sacrificing pen performance and reliability.